1. Field of Invention
This invention relates to variable drive ratio endless belt power transmission systems, specifically to pulley systems with expansible rim means adjustable in response to load requirements.
2. Description of Prior Art
Variable ratio belt drive power transmission systems utilizing expansible pulleys to provide variable pitch diameters on drive and driven elements are well known. Heretofore, most development effort has been invested in expansible cone pulleys and they are in wide use. Typically, these systems utilize axially adjustable pulley faces and control means to adjust the distance between faces, thereby changing the diameter at which a V-belt, normally, will traverse the pulley.
Those experienced in the art understand the limitations of such mechanisms. Although they have been developed to be reliable and efficient, their use has been restricted from the very low cost and size limited applications. As an example, the bicycle can be considered among the most critical of applications for a variable speed drive when cost, size, and weight limitations are considered. To date, there is no commercially viable, automatically adjustable, infinitely variable speed drive for the bicycle.
Attempts to develop an automatic transmission for the bicycle are many and varied, although none have displaced the existing manual shifting means. In U.S. Pat. No. 4,342,559, dated Aug. 3, 1982, Williams teaches the use of plates with guide grooves to control the radial position of pulley or sprocket segments and accomplish drive ratio variations. The pulley segments must slide freely between the control plates without jamming. Such systems are complicated and, hence, have proven too costly to impact the present market. Should they be developed to be cost competitive, the grooved guides can be expected to decrease durability and degrade performance. The added weight of the plates and related guide mechanisms is, likewise, a major detraction to these systems. They are also limited in the ability to be customized for a wide range of end users in one configuration.
In U.S. Pat. No. 4,642,070, dated Feb. 10, 1987, Walker utilizes toothed means to control the location of sprocket segments. As described, only the drive member changes diameter. Therefore, it must change by a large percentage in order to facilitate the range of drive ratios required for comfortable bicycle operation. Such a diametrical change would require very sturdy construction and its associated cost and weight. Also, a separate chain take up is required, again with extra cost and weight.
Thus, the above mentioned inventions have not been able to compete with the present manual shifting means in simplicity of operation, cost and weight. In addition to excessive cost and weight, they cannot be customized for a wide range of users. Durability is also seriously degraded as these complex systems are downsized to provide an adequate drive ratio range while meeting size and weight constraints.